The invention relates to a screen drum device for drying permeable webs of goods, especially textiles such as woven fabrics, knitted fabrics, tissue, nonwoven fabrics and the like in a housing having at least one rotatably mounted screen drum with a corresponding pumping device, such as a fan, at the front end which suctions off the gaseous drying medium from the inside of the screen drum and adds it back to the screen drum from the outside, said screen drum being associated with, on the outside, a stationary screen cover or a fixed cylindrical, screen body for equalizing the flow of the oncoming drying medium.
Screen drum dryers are known in various designs. The normal design is based on the registered utility model G 87 00 837, for example, according to which four screen drums are arranged in series next to each other, and a screen cover extends in a straight line above and below this row. The fans of each screen drum blow upward as well as downward, so that the recirculated air, after flowing through the screen covers, is also fed to the adjoining screen drums. The screen covers can also be bent centrally around the drums as shown in DE-A-39 05 001.
In addition to the bank of screen drums, dryers with only one screen drum are also known. There is, for instance, the design according to DE-A-43 25 915 with a screen cover arranged at an angle or, according to DE-A-30 06 758, with several screen drums in a device with screen covers arranged as segments of a circle or, according to DE-A-21 18 488, with a precisely centered screen cover on a device having, again, only one screen drum.
The best hole dimensioning, i.e., the optimum air permeability of the screen cover, is defined based on experience. Of importance is the uniform air supply to be achieved via the screen cover, which air supply is the prerequisite for the required temperature uniformity across the working width. It is known that the air permeability of the web of material is a function of the degree of moisture of the web. For clarification, one can refer to FIG. 5 in which several curves are entered of webs having varying degrees of moisture. This shows the increasing air permeability v(m/s) at varying pressure differences xcex94p (mm water column) measured at the material.
FIG. 6 is even more informative. There, the constant xcex6 from Bernoulli""s loss equation xcex94p=xcex6xc3x97p/2xc3x97v2 is plotted against angle xcfx86 at the screen drum (drying angle on the single-drum device). This constant xcex6 drops with increasing degrees of dryness. In the same way, the air speed v (m/s) through the material rises with increasing angle xcfx86 at the drum. These physical data are known, but are not taken into consideration in the design of the hood surrounding the screen drum.
The goal of the invention is to optimize the known screen drum dryer. An attempt is to be made to take into consideration the fact of the poorer air permeability of the wet material and also the improved air permeability of the drier material during the drying process and/or in the hood design.
In order to achieve is goal, it is suggested to provide the screen coverxe2x80x94as seen along its length in the housing in the direction of transport of the textile goodsxe2x80x94with a perforation whose open spaces per unit area increase directly with the degree of dryness of the textile goods to be dried.
With this design alone of the screen plate surrounding the screen drum inside the hood, the supply of drying air to every point of the rotating screen drum is quantitatively regulated in such a way that the air made available and distributed evenly around the screen drum in front of the screen plate by the fan can actually penetrate the goods at the respective points of the actual drying phase. FIG. 7 shows the actual air speed v through the material as a function of the drying phase. The design of the screen cover makes possible optimization of the maximum achievable air speed through the material as a function of the wrapping angle 0xc2x0 at the circumference at the inlet up to 270xc2x0 at the outlet of the goods as a function of dryness.
The screen drum system has the advantage that in the drum (exhaust) as well as in the hood (supply), constant pressure conditions prevail, i.e. independent of the wrapping angle at the circumference of the drum. Because of the measure according to the invention, there no longer exists the danger of drawing air into or blowing air out of the hood due to local negative air pressure (deficiency of air) or local excess air pressure (excess air). This can occur if, at the inlet of the screen drum dryer, the air uniformly supplied by the fan cannot penetrate the wet goods and therefore has to escape through the opening between the rotating screen drum and the screen plate as secondary air. The reverse is then true at the outlet of the entire screen drum structure, where the goods are more permeable to air based on their dryness, and the negative pressure in the screen drum is so great that it must draw in air through the above-defined opening in addition to the available drying air. Due to this fact, an undesirable air flow can also be created in the direction of the circumference of the drum, which is thus prevented. Another very important advantage of these measures according to the invention is the improved energy utilization.
The design of the screen plate can take on different forms. It is advantageous if the perforation holes across the surface of the screen cover maintain a constant diameter, but the distance between the holes becomes smaller with the increasing degree of dryness of the textile goods to be dried.
Since, over the length of the screen plate and the extent of the drying process, the screen plate, for economic reasons, cannot be continuously perforated with varying intervals, it makes sense to partition the screen cover over the length of the device into individual segments and to install in each segment a screen cover with a constant, free air throughput cross section, except perhaps for the subsequent screen cover segmentxe2x80x94in the transport direction of the textile goodsxe2x80x94which would be provided with a larger air throughput cross section. The same goal can also be achieved with two parallel screen plates, displaceable relative to each other.